Vacuum-Concentrating, Part 2

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DIY-Style

In my last post, I explained how vacuum-concentrating can condense flavor well below the boiling point of water, thereby leaving aroma compounds intact. Some Modernist chefs do this with a rotary evaporator, or rotavap for short. The only problem is that a full-sized version is a $40,000 piece of research equipment. Even a small one costs over $5,000. They’re fragile, and replacement parts aren’t cheap; they can leak in at least a dozen different places, requiring time to futz around and find the leak. They’re designed for laboratories, not for kitchens.

This isn’t to say that rotavaps aren’t useful for chefs. They are one of the few ways to capture distillate at temperatures below the boiling point of water. But if you want only the concentrate, rather than the distillate, there’s a much easier way to put together a vacuum-concentrating system. The photo below shows just how to do that (click the photo to enhance the image).

To build a vacuum-concentrating system, you need a few things:

1. First, you need a vacuum pump that can handle a lot of liquid. Many cheap vacuum pumps use oil, but if you pull water vapor through that oil it will emulsify, gum up, and damage the pump. Make sure to get a water-recirculating aspirator pump with a capacity of about 10 liters. This looks like a beer cooler, but inside there’s a pump that circulates water. As the water flows by the little orifice in the nozzle, it creates a venturi effect, creating a vacuum. Because they’re sold to laboratories (which are less sensitive to price), new ones can cost more than $1,000. If you’re mechanically inclined, you can take a trip to any major hardware store and get everything you need to build your own. If you look around on eBay for recirculating aspirator pumps, however, you’ll find a lot of these for far less than the one linked to above.

Your pump should be able to pull 5-40 mbar (0.07-0.58 psi), depending on water temperature. The colder the water, the stronger the vacuum will be. To maintain a cold temperature, keep ice floating in the water bath while it’s circulating.

An aspirating nozzle, which has a little side arm that you can screw onto your faucet, is an even cheaper alternative. Vacuum strength will depend on how fast the tap water is flowing as well as the water temperature. The downside to these devices is that you throw away tens of gallons of water. That water goes down into the sewage to be reused, but it can add up. If you vacuum-concentrate a lot, a recirculating pump probably makes sense financially, but if you just want to try it, you should go with the faucet aspirator because you’ll save a few hundred dollars.

2. The next thing you need is a vacuum flask, sometimes called a side-armed Erlenmeyer flask. They come in myriad sizes, from a few hundred milliliters (about one cup) up to tens of liters or more. For home use, 2-5 liters is optimal.

3. You also need rubber vacuum tubing. Most flasks require a hose with an inner diameter of 5/16 in. You can find this sold by the meter in a well-supplied auto parts store, or online.

4. Your flask will need a size-appropriate stopper, which is sold separately. For example, a 2-liter flask takes a number 9 stopper.

5. You need a Teflon-coated magnetic stir bar. This will work in conjunction with item #6 below, and should be about 2 in long.

6. To go with the magnetic stir bar, you need a magnetic stirring hot plate, about 6-7 sq. in. Again, because this is a piece of lab equipment, it’s more expensive than you’d guess. Luckily, eBay is just brimming with them. Digital ones cost more, but analog is just fine.

This handy gadget not only heats the plate, but also creates an alternating magnetic field that causes that stir bar inside your glass flask to spin. Once it gets going fast enough, the stir bar creates a vortex, which expands the surface area of the liquid and thus increases the rate of evaporation. The vortex also encourages nucleation. When liquid is in a smooth glass flask, it tends to boil quite violently because there are few nucleation sites on which bubbles can form. In such situations, the temperature of the liquid can actually become super-heated, rising a couple of degrees above its boiling point. You may have seen this phenomenon if you’ve ever heated a mug of water in the microwave and noted that it barely bubbled at all until you dropped a spoon in it, at which point the liquid suddenly boiled all at once. When super-heating occurs inside a stoppered flask, a huge bubble can burst to the surface so violently it can actually cause the flask to jump off the plate and shatter. Stirring the liquid creates little bubbles that serve as nucleation sites, so the liquid boils steadily and more safely.

A magnetic stirring setup creates a vortex that assists boiling.

The key idea here is that the liquid in the flask can never be hotter than its boiling point, which is determined by the strength of the vacuum. This is just like boiling water on a gas burner because while the burning gas beneath it is thousands of degrees, the water in the pot is not above 100 ?C / 212 ?F. Turning the heat up higher will make it boil faster, but it doesn’t make it boil hotter, so your flavor compounds remain intact. You want this hot enough so that it boils fast enough to get the evaporation to make it worthwhile, to get the job done. If you go too fast, the pump can’t keep up and the pressure starts to rise, so then the temperature rises a little. We tend to set the hot plate to about 205 ?C / 400 ?F. If the water is cold enough in the pump, it will boil away at 26 ?C / 80 ?Fa warm swimming pool, but not warm enough to change delicately flavored liquids, such as a citrus juice. You could set your hot plate as low as 150 ?C / 300 ?F, but you’d be surprised, you almost never want it to go lower than that for a reasonable rate of evaporation.

Nathan Talks Cookbooks on Eater.com

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One thing we’ve learned from working with Nathan Myhrvold is that if you get him talking about a topic that interests him (and there are many, many, many topics to choose from), he’ll go on and on until someone comes and rushes him off to another meeting. You’ll probably learn something in the process, too, even if it’s a subject you think you know everything about. Recently, Eater.com‘s Paula Forbes got him talking about his cookbook collection: which ones are his favorites, which he’s most sentimental about, and why he doesn’t think much of the barbecue books out there.

A few of the cookbooks Nathan keeps at The Cooking Lab. Photo courtesy of Paula Forbes.

Click here for the whole interview.

Is Liquid Nitrogen Safe?

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At the beginning of the MC project, Nathan set out to dispel many of the myths surrounding cooking, yet some common misconceptions about liquid nitrogen still persist. Sometimes we get questions like Is liquid nitrogen dangerous? Will it hurt you? Or, You can’t cook with liquid nitrogen! It’s poisonous!

The truth is, liquid nitrogen is completely inert except for its extreme temperature. It will cause any metal it comes in contact with to become freezing cold, but wearing dry gloves is enough to protect your hands from creating a “tongue stuck to the flagpole” scenario. The liquid nitrogen itself will evaporate before it contacts your skin due to the Leidenfrost effect (see video below).

Actually, liquid nitrogen pales in comparison to the dangers involved in most applications of fryer oil or even sugar. Fryer oil is extremely hot; it spills, it splatters, it splashes. Any cook who works frequently with deep fat fryers gets burned all the time. You get little blisters on your arms and hands when heating oil. The day we shot our wok cutaway photo, Max got all sorts of burns on his arms from tossing the phad Thai and oil so many times.

By the end of this shoot, Max’s arms were full of tiny burns from the hot oil.

When it comes to kitchen burns, sugar is enemy number one. Anyone who has had a close encounter with hot caramel knows that you really don’t want this stuff on your skin. If a little bit of the hot caramelized sugar lands on your hand, your first reaction is to rub it, which leads you to smear it onto your other hand. It just sticks everywhere, and you end up burned all over.

I’ve been working with liquid nitrogen in the kitchen for about five years now. I’ve dipped my bare hands in it, spilled it, splashed it, but never been hurt by it. I’m not saying you should go ahead and goof around with it, but you should give it a chance without fear. Go ahead and try it! It’s great for all sorts of applications. Just put on gloves, wear long pants so that it can’t drip into your shoes if you spill any, and don’t eat food until you’re sure the nitrogen has boiled off of it. (For a more complete discussion, see “Safe Handling of Cryogens,” page 2·464-466 in Modernist Cuisine.)

A number of recipes in Modernist Cuisine use liquid nitrogen to achieve special effects, from firm coating gels to foie gras torchon, from shrimp and grits to buttermilk biscuits. And, of course, we love Nathan’s method of cryofrying meat, which is to cook meat sous vide, then dip it in liquid nitrogen, and finally deep-fry it quickly to get a really nice, Maillardized outer crust with a rare or medium-rare interior. We use this technique in our mushroom cheeseburger recipe. And again, it’s really the hot oil from the deep fryer that you have to watch out for in that recipe.

Wearing gloves when handling liquid nitrogen protects your hands from the cold temperature of the metal container.

Although it’s not hard to handle liquid nitrogen safely, it is also not completely without risk. In fact, I just happen to be one of the few people in the world who have actually had a traumatic experience with the substance. I once used liquid nitrogen at a dinner for some guests and afterward was transporting a Dewar of the stuff in the back of my SUV. Although the Dewar was in perfect condition, some of the dinner guests had been playing with it and hadn’t refastened the lid. I didn’t realize that, and as I was heading up a hill, the Dewar fell over. Liquid nitrogen has a very low viscosity, so it is thinner than water and flows like crazy. It quickly spread all over the bottom of the car, and as it boiled off furiously, the car rapidly filled with vapor. It also got really cold, and I couldn’t see out of my rear view mirror or rear window. It was like driving through the densest fog–but the fog was inside the car!

The correct way to transport liquid nitrogen.

I pulled over and got out of the car as fast as I could. As the nitrogen evaporates into gas, it displaces oxygen in the air, so if a lot of it spills in an enclosed space it can create a suffocation risk. Emerging from the car, I looked back and saw white fog pouring out from every opening. Luckily, our photographer, Ryan Matthew Smith, was behind me and also pulled over. We opened the hatch of the SUV to get the Dewar out, in case it was still leaking. I heard the plastic in the car crackling as it warped from the intense cold.

When it was all over, I was surprised to find that despite the large size of the spill, it didn’t cause any permanent damage. If the Dewar had been filled with super-hot fryer oil instead of ultra-cold liquid nitrogen, it would have been a different story.

A New Recipe and Our Cutest Video Yet: Olive Oil Gummy Worms!

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We use fishing lure molds to make our creepy crawlers.

A special helper came to The Cooking Lab recently to help us prepare for Halloween. Find him guest-starring in our newest recipe video as he helps his dad, one of our culinary research assistants, Johnny Zhu, make olive oil gummy worms and “dirt.” We also have the recipe, along with photos and tips.

Have you signed up yet to receive our emails? You can get our recipes a few days earlier than we post them here when you check “receive emails” on your profile page. Register here to get started!

Max Will Demonstrate Modernist Techniques at Seattle’s Book Larder

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Book Larder, Seattle’s newest food-focused bookstore, is offering a special package that includes both a copy of Modernist Cuisine and two tickets to an event with MC coauthor Maxime Bilet for a discounted price of $550. That’s $75 off the list price! Please join Max at Book Larder on Thursday, February 2, 2012, at 6:30 p.m. for a demonstration of techniques from the book, insights into MC’s creation, and a Q&A session. For more information, click here to visit the page on Book Larder’s website.

Vacuum-Concentrating, Part 1

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The Lower, the Better

Concentrating flavor is one of the most basic yet important tasks in cooking. From a technical point of view, concentrating generally means evaporating off a solvent while leaving behind as many flavorful molecules as possible. In the kitchen, the solvent is usually water, but sometimes is alcohol. Rarely is it anything else; although fats and oils are edible solvents, you’ll create a spectacular fire if you try to vaporize them.

The traditional way of concentrating flavors relies on heating the liquid to its boiling point.

To get the job done in any reasonable length of time, you must raise the temperature of the solvent to very near its boiling point. The downside to this is that water boils at about 100 °C / 212 °F (the exact temperature varies with altitude and weather conditions), which is often hot enough to dramatically alter many of the flavors you’re trying to concentrate. Sometimes those alterations are exactly what you want: simmering a meat stock for hours plays a crucial role in creating the rich flavor of a traditional demi-glace, for example. But in many cases, the new flavors aren’t so delicious. As a rule of thumb, foods that people usually eat raw are likely to suffer from the high temperatures that reduction requires. When you concentrate an orange juice, for instance, you lose its vibrancy, and it ends up tasting like… well, cooked orange juice.

It turns out there is an alternative way to concentrate these delicate kinds of flavors without ruining them. Increasing pressure raises the boiling point of water (as happens in a pressure cooker), and conversely decreasing pressure lowers the boiling point of water. So the lower the pressure (the stronger the vacuum), the lower the boiling point. In fact, it’s entirely possible to reduce the pressure so far that ice-cold water will boil. A couple years ago in our research kitchen, we used a chamber vacuum sealer to freeze liquid nitrogen solid!

A rotary evaporator offers unparalleled flexibility in creating vacuum-concentrated juices and sauces.

When talking about vacuum pressure, it’s both convenient and illustrative to quantify the pressure in units of millibars (mbar). At sea level, the standard atmospheric pressure is 1,013.25 mbar and the boiling point of water is 100 °C / 212 °F. Take a trip to the mile-high city of Denver and the pressure drops to 805 mbar, and water boils at 93.7 °C / 200.7 °F. That’s not too great a difference, but a vacuum-concentration setup can reduce the pressure surrounding your pot of liquid all the way down to 55 mbar, enough for it to come to a boil at the perfectly pleasant room temperature of 20 °C / 68 °F. That moderate temperature will not destroy any delicate and fresh-smelling aroma compounds.

And more of those compounds will stay in the food, rather than being flung into the air as happens during traditional stove-top reduction. Now it’s true that lowering the boiling point of water also lowers the boiling point of other volatile molecules, so even vacuum reduction does throw away some of those aromas (which make the kitchen smell so nice). But more of them will remain where you want them–flavoring the food–than if you just turn up the burner to drive off the liquid.

It’s easy to imagine all kinds of dishes that benefit from flavors concentrated at low temperatures. At The Cooking Lab, we vacuum-concentrate granny smith apple juice, to preserve its fresh, tart flavor as well as its bright green appearance. Vacuum-reduced wine-based sauces are also interesting because you can boil out both the ethanol and water at very mild temperatures. Personally, I like to use a simple setup I have at home to prepare cocktails with vacuum-concentrated infusions and tinctures.

The results are always very different than anything you’ve had before. Actually, it’s a bit hard to describe these flavor profiles because few people have tasted anything like them before. Until now no one has come up with an easy way to vacuum-concentrate in the kitchen.

In my next post, I’ll show you how to build your own relatively simple and inexpensive vacuum-concentrating setup with the help of a little Google-fu. In the meantime, check out the table below for a range of concentrating strategies, all of which are covered in Modernist Cuisine. (Click on the table for a larger version suitable for printing.)

Bill Gates on Nathan Myhrvold and Modernist Cuisine

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Can Science Improve Cooking? from bgC3 on Vimeo.

News of the “Former Microsoft CTO Publishes Giant Cookbook” has been heralded around the world since the launch of MC. But there’s no denying that, despite having a PhD and working with Stephen Hawking after turning just 23, Nathan Myhrvold got his start when Bill Gates hired him as Microsoft’s first chief technology officer. Watch Gates as he explains Modernist Cuisine and gives a little insight into his former colleague and lifelong friend.

For more from Gates’s perspective, check out the article and slide show on the gatesnotes.

See Nathan Alongside Wylie, Emeril, and Others in South Beach

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Nathan Myhrvold will join several esteemed chefs to pay tribute to chef Charlie Trotter and winemaker Piero Antinori during the Food Network South Beach Wine and Food Festival on February 25, 2012, in Miami, Florida. Anthony Bourdain will host the event, which will showcase the talents of the world’s most renowned wine and spirits producers, chefs, and culinary personalities such as:

  • Marchese Piero Antinori
  • Anthony Bourdain (“No Reservations” on the Travel Channel)
  • Frederic Delaire (Loews Miami Beach Hotel)
  • Wylie Dufresne (wd ?50)
  • Michelle Gayer (Salty Tart Bakery)
  • Emeril Lagasse (Emeril’s)
  • Nathan Myhrvold (Modernist Cuisine)
  • Patrick O’Connell (The Inn at Little Washington)
  • Charlie Trotter (Charlie Trotter’s)
  • Norman Van Aken

In particular, Nathan will prepare hors d’oeuvres to be passed during the cocktail reception.

Tickets go on sale October 24, 2011. For more information, click here.